The subject matter of the instant invention is a device and a method for measuring the contact angle of liquid droplets on material surfaces.
The wetting capability of surfaces of metal and plastic parts is an important quality characteristic before subsequent process steps such as coating or gluing. Inadequate wetting capability of the metal and plastic surfaces leads to a loss of quality, so the required functionality is no longer ensured. The causes can be impurities and insufficient surface energy of the parts. In the case of metal parts, the contamination of the surface can be evaluated with a cleanliness check via fluorometry. The fluorometry technique is not suitable for a cleanliness check of plastic surfaces, though, because they usually have a high level of autofluorescence. A cleanliness check of the plastic surfaces can be done through the wetting check under certain circumstances. A good wetting capability is achieved when the surface energy of the part is greater than the surface tension of the coating material. The surface energy of ideal clean parts can be specifically increased or changed with surface pretreatment, for instance with a plasma treatment or by coating them with special functional layers. The desired effect of this pretreatment is to be checked on a regular basis within the framework of the quality assurance, in order to ensure a high level of process reliability.
The state of the art with regard to a simple check of the wetting capability of the surfaces currently involves the use of test inks. The test-ink method is a quick check that provides the user with an approximate value for the surface energy of the part to be checked. In the process, test liquids with a defined surface tension are applied to the part to be checked. If the surface energy of the metal or plastic part is greater than the surface tension of the test ink, the applied ink spreads out. In the reverse case, when there is a greater surface tension of the test ink, the test liquid contracts and irregular droplets are formed. This method can only be used in a limited way because the maximum surface energy of the test specimen that can checked is limited to 72 mN/m by the largest possible surface tension of a test ink. Furthermore, the following drawbacks arise when the test inks are used to check the wetting:                The test inks are applied in a two-dimensional fashion, making the surfaces to be examined very dirty.        The chemicals used in the test inks are poisonous and reprotoxic in part. Special precautions are to be taken during use and disposal.        Test inks only have a limited shelf life.        Test-ink series of different liquid mixtures are not comparable due to different inter-molecular interactions.        
Measurement of the contact angle of a liquid on the solid is a further possibility for checking the wetting capability of metal and plastic surfaces. The contact angle measuring instruments currently offered on the market are based for the most part on the sessile drop method. This involves the evaluation of the drop contour of a liquid drop that is previously put in place by taking a shadow image with the aid of image processing. The surface energy of the solid can be precisely determined via the contact angle with this method. The commercial measuring instruments based on this method involve in principle a two-part structure. The measurement module is comprised of a proportioning unit for applying a liquid drop with a specific volume to the surface and an optical system made up of a camera with an objective lens and lighting. The optical axis is aligned in parallel with the surface of the solid. The shadow image that is generated in that way is recorded and transferred to a connected PC. Software determines the drop contour and the contact angle from the shadow image and then calculates the surface energy. There are mobile contact angle measuring instruments as well as stationary laboratory measuring instruments. Well known suppliers are, for example, the company Krüss with the measuring instrument GH11, the company DataPhysics with the measuring instrument PCA 100M, the company Fibro System AB with the measuring instrument PGX and the company Sindatek with the measuring instrument Sindatek 100P. The drawbacks of these contact angle measuring instruments are                the high price,        the large contact surface that is required (surfaces with complex geometries cannot be checked),        the very low level of automation (user intervention is necessary) and        the limited mobility in the process check because of the required use of a PC.        
An instrument for determining the concentration of a material in a solution influencing the contact angle is described in EP 1 729 109 A1. Two cameras are used here that record the contact angle of a solution droplet on the material surface at right angles to one another.
A similar, less automated approach is described in U.S. Pat. No. 5,268,733. The shadow of the drop is projected onto a surface perpendicular to the material surface here via a light source that emits light in parallel with the material surface. This surface has a scale, so the contact angle can be directly read on the surface. This structure obviously involves a solution that can only be used for laboratory measurements. As can be seen, a complex adjustment of the specimen surface is necessary vis-a-vis the scaled shadow surface. Industrial use requires, however, a quick measurement with robust designs.
A further drawback of the known contact angle measuring instruments that are based on the principle of EP 1 729 109 A1 is that the material surface tends towards mirroring under certain circumstances. Because of that, it is difficult for the evaluation software that is used to correctly determine the course of the material surface. Faulty calculations could come about that can only be recognized and corrected with a human follow-up at present.
DE 197 54 765 C1 describes a contact angle measuring instrument in which a drop of test liquid is set down on a surface. This drop is illuminated with a light beam running in parallel with the surface that is redirected by a prism. The light is redirected by a second prism into a camera after passing through the drop. The digitalization and, after that, the calculation of the contact angle take place in the camera. Recording the contact angle with sufficient precision is difficult with this design, because it is naturally directly formed on the surface and has to be captured by the second redirecting prism in its contact with the surface. If damage to the edge of the prism or dirt exists here, a precise measurement is hardly possible.